1. Field of the Invention
The present invention relates generally to a suspension system for a vehicle. More specifically, the invention relates to an automotive suspension system with improved cornering characteristics. Further particularly, the invention relates to a control system for an automotive suspension system, which actively controls load distribution at respective vehicular wheels for better cornering characteristics.
2. Description of the Background Art
Generally, a typical construction of a suspension assembly comprises a suspension coil and shock absorber and is interposed between a vehicle body and a suspension member supporting a vehicular wheel, in order to constitute an automotive suspension system with the suspension member. The suspension coil spring generally resists against lad applied to maintain the vehicle body and the road wheel in a predetermined positional relationship to each other. On the other hand, the shock absorber is intended to damp or absorb vibrations transmitted between the vehicle body and the road wheel. The automotive suspension system may be further provided with a roll-stabilizer for suppressing vehicular rolling motion.
Some of the many kinds of automotive suspension systems include hydraulic circuits associated with fluid chambers in hydraulic shock absorbers for the controlling balance between the fluid pressures in the fluid chambers according to the relative displacement between the vehicle body and road wheel. Such hydraulic circuits include a fluid pressure source supplying working fluid at a given pressure to the circuits, and pressure control valves. The pressure control valves hydraulically connect and disconnect the fluid pressure source with the fluid chambers of the hydraulic shock absorbers for controlling pressure supply. The pressure control valves are controlled by an electric or electronic control system which switches the various valve positions to selectively introduce or drain fluid pressure into or from the fluid chambers so as to generate a damping force which suppresses vehicle body and road wheel vibrations.
These conventional positively controlled suspension systems encounter various defects and have not been at all satisfactorily capable of suppressing vibrations or bouncing of the vehicle body in order to ensure riding comfort. Specifically, conventional systems produce damping force by means of an orifice in the hydraulic circuit. However, due to flow resistance through the orifice, fluid pressure differences between the fluid chambers in the shock absorber cannot be easily balanced. When the balance is disturbed, the shock absorber tends to transmit vibration of the road wheel to the vehicle body which degrades riding comfort.
Therefore, it is an object of the present invention to provide an automotive suspension system which has a hydraulic damping force control circuit capable of effectively suppressing vehicle vibrations, thus ensuring riding comfort.
In order to provide this improvement, a positively or actively controlled automotive suspension system has been proposed in European Patent First Publication No. 01 93 124, published on Sept. 3, 1986, and assigned to the common owner of the present invention. The proposed positively controlled automotive suspension system comprises a hollow cylinder defining a chamber, a piston thrustingly received within the chamber of the cylinder and defining therein a first and second fluid chambers, both filled with a working fluid, the piston being free to move axially with the chamber, a fluid pressure source, a hydraulic circuit connecting the first and second fluid chambers and the fluid pressure source, a pressure control valve disposed within the hydraulic circuit and adapted to selectively establish and block fluid communication between the first and second fluid chambers and the fluid pressure source, means responsive to relative displacement between a vehicle body and road wheel assembly out of a predetermined normal range, for controlling the pressure control valve so as to adjust the fluid pressure in the first and second fluid chambers in order to adjust the relative distance between the vehicle body and the road wheel assembly back to within the predetermined normal range, and means responsive to bounding and rebounding motion of the road wheel relative to the vehicle body, for controlling the pressure control valve so as to adjust the fluid pressure in the first and second fluid chambers to assist smooth displacement of the piston within the cylinder thereby absorbing bounding and rebounding energy which would otherwise be transmitted to the vehicle body.
On the other hand, as is well known, driving torque to rotate the road wheel acts on the tread of the road wheel as longitudinal force. On the other hand, in cornering, a lateral force acts on the tread of the road wheel due to centrifugal force. The combined force of the driving torque and the lateral force determines the cornering characteristics of the vehicle. Namely, cornering force is the counter force against the combined longitudinal and lateral forces as set forth. Generally, the magnitude of this counter force to be generated is determined by road/tire friction. The magnitude of the road/tire friction is variable depending upon the load applied to the road wheel. Therefore, the combination of the combined force and the counter force at each vehicular wheel will determine the cornering characteristics of the vehicle.
In other words, the maximum counter force to be generated at each wheel is determined by road/tire friction. The distribution of the friction magnitude at the respective road wheel is variable depending upon the shift of a gravity center during cornering due to the change of load distribution. When the lateral force becomes greater than the maximum lateral component of the counter force, the tire loses traction to cause a lateral slide to increase over-steer characteristics. On the other hand, as long as the road/tire traction is maintained, under-steer characteristics may increase according to increasing driving torque as the longitudinal force. This prevents the vehicle from handling the corner at high speed. Therefore, in order to optimize cornering characteristics of the vehicle at relatively high speed, balance of the combined force of the longitudinal and lateral forces and the counter force is required. However, in order to obtain such optimum cornering characteristics, high driving techniques, such as delicate accelerator pedal control and steering operation synchronized with accelerator pedal control is required.